For the past 150 years, lighting technology has been primarily limited to incandescent and fluorescent tube light sources. These sources were typically the only ones that produced sufficient illumination in an acceptable spectrum. However, both incandescent and fluorescent bulbs are relatively inefficient in converting electrical energy into light. While derivative technologies such as high-intensity discharge lamps (HID) have emerged, none have achieved energy efficiencies exceeding 25%, with incandescent lighting achieving an efficiency of less than 2%.
Incandescent and fluorescent bulbs are also fragile and relatively not compact. The fragility results from the requirement of creating a significantly less than atmospheric pressure environment inside the bulbs. The volume of the bulbs makes them non compact. Fluorescent bulbs also generally require ballast to ignite, raising the cost of the bulbs.
One alternative being investigated to replace the traditional incandescent and fluorescent lighting sources is known as Solid State Lighting (SSL). Solid state lighting based upon solid state electronic devices, such as light-emitting diodes (LEDs), organic light-emitting diodes (OLED), or polymer light-emitting diodes, as sources of illumination rather than electrical filaments or gas.
Light emitting diodes are in widespread use, but not as general lighting sources. Commercial LEDs debuted in the 1960s. Initial LEDs were red in color, with yellow and orange variants following soon thereafter. White light may be created by combining the light of separate LEDs (red, green, and blue), or by creating white LEDs through doping.
Unlike traditional lighting solid state lighting creates visible light with virtually no heat or parasitic energy dissipation. Solid-state light sources are also much more durable, proving greater resistance to shock, vibration, and wear, and possess impressive operational lifespan significantly. A significant limitation of solid state lighting, including LEDs, is their inability to produce broad spectrum light, namely white light. White light is desirable for general lighting purposes as human eyesight finds white light to be comfortable.
Efforts have been made to produce white lights from LEDs. One technique is known as color mixing. Color mixing involves utilizing multiple LEDs in a lamp and varying the intensity of each LED to produce white light. The lamp contains a minimum of two LEDs (blue and yellow), but can also have three (red, blue, and green) or four (red, blue, green, and yellow). In the color-mixing method, degradation of different LEDs at various times can lead to an uneven color output. For white light to be produced that spans the visible spectrum (red, green, and blue), LEDs must produce the component narrow band emissions in the correct proportions.
Wavelength conversion is the other general technique. It is preferable to color mixing, as wavelength conversion results in a true white emitting LED. Wavelength conversion involves converting some or all of the LED's output into visible wavelengths. Wavelength conversion methods used to accomplish this feat include:
1) Blue LED & yellow phosphor—This is considered the least expensive method for producing white light. Blue light from an LED is used to excite a phosphor which then re-emits yellow light. This balanced mixing of yellow and blue lights results in the appearance of white light.
2) Direct Bandgap Materials A certain size distribution (around˜3.7 nm) of InP nanoparticles, a direct bandgap material, which produce sharper lines of luminescence for a certain particle size, has been proposed for incorporation in phosphorous LEDs to alleviate the red problem. Testing or implementation has not been achieved yet.
3) Blue LED & several phosphor—This is similar to the process involved with yellow phosphors, except that each excited phosphor re-emits a different color. Similarly, the resulting light is combined with the originating blue light to create white light. The resulting light, however, has a richer and broader wavelength spectrum and produces a higher color-quality light, albeit at an increased cost.
4) Ultraviolet (UV) LED & doped phosphors (red, green, & blue)—The UV light from a UV LED is used to excite different phosphors, which are doped at measured amounts. The colors are mixed resulting in a white light with the richest and broadest wavelength spectrum.
5) Blue LED & quantum dots—A process by which a thin layer of nanocrystal particles containing 33 or 34 pairs of atoms, primarily cadmium and selenium, are coated on top of an LED. The blue light excites the quantum dots, resulting in a white light with a wavelength spectrum similar to UV LEDs. This method is unlikely to prove successful for several reasons. First, the production of the nanomaterial is very expensive, for example being nearly 200 fold more than organic pigments or Si nanoparticles. Also, the cadmium and selenium nanocrystal particles are not available in a uniform size distribution, making it difficult to control color mixing.
Both of the color mixing and known wavelength conversion techniques face significant limitations that are barriers to widespread adoption as lighting sources. The current manufacturing processes for the wavelength conversion techniques above are immature and not cost-effective. There are also problems with the phosphor conversion model. These techniques do not emit broad enough wavelength spectrum of light, as they lack a red component. They also have absorption and emission that can't be tuned, and possess inflexibility of form resulting from the need for the phosphor to be placed in the emission path of the LEDs and have the phosphor to have its emission directed in a desired direction. Phosphor films are also known to have appreciable reflectivity, which causes a non-negligible fraction of the LED light to be reflected back toward the LEDs, causing heating effects that can make driving the LED more difficult and can create other difficulties, necessitating more complex and rigorous heat dissipation designs. Also, since phosphors respond more strongly to the blue portion of the LED light, the transmitted UV portion can be hazardous to human vision. With systems operating at high intensity, efforts should be made to incorporate UV blockers. The same problems can be said about organic pigments.
One recent concept that employed UV blockers for safety purposes involves TiO2 nanoparticles added to the active layer to attenuate the UV, while automotive grade heat resistant organic pigments were used to down convert the blue light. The use of TiO2 is necessitated by the fact that the pigments are not sensitive to UV.